Geologic units in Mills county, Iowa

Douglas Group (Upper Pennsylvanian, upper Missourian-lower Virgilian) at surface, covers 31 % of this area

Interval includes Stranger, Cass, Lawrence formations. Primary Lithologies: shale and mudstone, light to dark gray, part silt laminated. Secondary Lithologies: limestone, fossiliferous, part coated grain; siltstone to very fine sandstone; pedogenic mudstone, part red-brown, part calcareous/nodular. Minor: coal, carbonaceous shale; black shale. Maximum thicknesses 90-135 ft (27-41 m).

Shawnee Group (Upper Pennsylvanian, lower-middle Virgilian) at surface, covers 28 % of this area

Interval includes Oread, Kanwaka, Lecompton, Tecumseh, Deer Creek, Calhoun, Topeka formations. Primary Lithologies: limestone, fossiliferous, part coated grain/Osagia; shale/mudstone, light to dark gray. Secondary Lithologies: pedogenic mudstone, part calcareous/nodular; siltstone to very fine sandstone; black shale, phosphatic. Minor: chert; coal, coaly shale; red-brown mudstone. Maximum thicknesses 190-220 ft (58-67 m).

Dakata and Windrow Formations (“Mid” Cretaceous; upper Albian-upper Cenomanian) at surface, covers 16 % of this area

Dakota Formation widespread in western Iowa, lower sandstone-dominated Nishnabotna Member, upper mudstone/shale-dominated Woodbury Member. Correlative Windrow Formation found as erosional outliers in northeastern and north-central Iowa. Primary lithologies: sandstone, quartzose, very fine to medium grained; mudstone/shale, light to dark gray, variably silty-sandy, noncalcareous (Woodbury Mbr.). Secondary lithologies: sandstone, medium to very coarse grained, part pebbly to gravelly, locally cemented by iron oxides (Nishnabotna Mbr., Windrow Fm.), gravel, quartz and chert clasts; siltstone; mudstone, red, pink, yellow-brown, black (carbonaceous). Minor: lignite; siderite (concretions, pedogenic sphaerosiderite pellets, cemented siltstone); massive iron ore, silty to sandy (Windrow Fm.). Maximum thickness Dakota Fm. 500 ft (150 m), commonly 100-300 ft (30-90 m); Windrow Fm. 40 ft (12 m).

Lansing Group (Upper Pennsylvanian, upper Missourian) at surface, covers 10 % of this area

Interval includes Plattsburg, Vilas, Stanton formations. Primary Lithologies: limestone, fossiliferous; shale/mudstone, light to dark gray. Secondary Lithologies: limestone, coated grain/Osagia; mudstone to siltstone, gray. Minor: black shale, phosphatic; mudstone, red-brown. Maximum thicknesses 45-78 ft (14-24 m).

Wabaunsee Group (Upper Pennsylvanian, middle-upper Virgilian) at surface, covers 7 % of this area

Interval includes Howard, White Cloud, Scranton, Bern, Auburn, Emporia, Willard, Zeandale, Pillsbury, Stotler, Root formations. Primary Lithologies: shale, light to dark gray; mudstone, gray, part laminated silty to sandy. Secondary Lithologies: limestone, fossiliferous, part coated grain/Osagia; pedogenic mudstone, part red to maroon, part calcareous/nodular; siltstone to very fine sandstone. Minor: black shale; coal. Maximum thickness 320 ft (98 m) in Fremont County.

Kansas City Group (Upper Pennsylvanian, middle Missourian) at surface, covers 7 % of this area

Interval includes Cherryvale, Nellie Bly, Dewie, Chanute, Iola, Liberty Memorial, Wyandotte, Lane formations. Primary Lithologies: limestone fossiliferous; shale/mudstone, light to dark gray. Secondary Lithologies: limestone, coated grain/Osagia. Minor: mudstone, red-brown to green-gray; siltstone; black shale, phosphatic; thin coal, carbonaceous shale. Maximum thicknesses 100-140 ft (30-43 m).

Bronson Group (Middle to Upper Pennsylvanian, uppermost Desmoinesian-lower Missourian) at surface, covers 0.6 % of this area

Interval includes Pleasanton, Hertha, Elm Branch, Swope, Galesburg, Dennis formations. Primary Lithologies: limestone, fossiliferous; shale/mudstone, light to dark gray. Secondary lithologies: mudstone to siltstone, gray to green-gray; limestone, coated grain. Minor: black shale, phosphatic; red/maroon mudstone; coal; chert. Maximum thicknesses 100-130 ft (30-40 m).

Lansing Group (Late Pennsylvanian - Missourian) at surface, covers < 0.1 % of this area

Limestone and shale. Limestone is dark gray to light gray, very thin bedded to massive, sandy fossiliferous, and cherty. Shale is dark gray to light gray, maroon, black, fossiliferous, and calcareous. Approx. max thickness 75 ft

Kansas City Group (Late Pennsylvanian - Missouria) at surface, covers < 0.1 % of this area

Limestone and shale. Limestone is dark gray to light gray, brownish gray, very thin bedded to massive, argillaceous, fossiliferous; contains very thin layers of chert, pyrite crystals, and small flakes of mica near base. Shale is dark gray to light gray, greenish gray, red, and black; locally slighty sandy, calcareous, carbonaeous, fissile, and fossiliferous. Approx. max thickness 200 ft